How does soft tissue distribution affect anteversion accuracy of the palpation procedure in image-free acetabular cup navigation? An ultrasonographic assessment.

Navigation of the acetabular cup in total hip replacement (THR) is used to improve the reproducibility of acetabular component positioning. When the palpation of anatomic landmarks, which is necessary to determine the pelvic coordinate system, is performed epicutaneously, the question as to how uneven soft tissue distribution can influence navigation accuracy arises. To obtain data, the questionable soft tissue thickness was measured in 72 patients scheduled for THR. In addition, distances between the landmarks were recorded. On the basis of this information, we were able to calculate the expected misinterpretation of the anteversion given by a navigation system for each patient. The calculations suggest that a navigation system would have underestimated the anteversion on average by 2.8 degrees +/- 1.8 degrees. The median of anteversion misinterpretation was 2.4 degrees and its 95% confidence interval was calculated to be 2.2 degrees -3.0 degrees. No correlation with substantial significance between anteversion misinterpretation and the patients' biometrical data could be found. According to the current knowledge, acetabular cups in THR should be positioned within a range of 30 degrees -50 degrees of inclination and 10 degrees -30 degrees of anteversion. In comparison with these permitted +/- 10 degrees windows, the amount of misinterpretation that was found due to uneven soft tissue distribution seems to be acceptable.